Illustration par défaut

Ubiquity of interplanetary nanodust particles from Cassini observations

11 avril 2015

An international team, led by astronomers from Paris Observatory, has measured for the first time the distribution of nanodust particles in the interplanetary medium between the orbits of Earth and Jupiter. This result confirms that these particles are released near the Sun and ejected outwards at several hundreds of kilometers per second, as predicted by theoretical models.

These nanoparticles bridge the gap between molecular ions and dust grains. They were detected for the first time in the solar wind at 1 Astronomical Unit from the Sun (1 AU is the Sun-Earth distance) by the STEREO probes in 2009. Their small size gives them peculiar properties. Since a large proportion of their atoms are lying at their surface, they interact in a privileged way with their environment. And their electric charge is high enough for their dynamics to be governed by electromagnetic forces. In this way, nanodust particles can be accelerated by the magnetized solar wind and expelled from the inner heliosphere to large distances at nearly 400 km/s.

When such a fast nanoparticle hits a spacecraft, the impact produces a microcrater whose material is vaporized and ionized, producing an expanding plasma cloud (Figure 1).

Figure 1. Principe de la détection de poussières rapides par un instrument radio à bord d’une sonde spatiale (crédit : Meyer-Vernet et al. 2010, DOI : 10.1063/1.3395912)
Figure 1. Principe de la détection de poussières rapides par un instrument radio à bord d’une sonde spatiale (crédit : Meyer-Vernet et al. 2010, DOI : 10.1063/1.3395912)

Figure 1. Basics of the detection of fast dust particles with a wave instrument onboard a space probe. (credit : Meyer-Vernet et al. 2010, DOI : 10.1063/1.3395912)

The plasma electric charges produce a transient signal that can be detected by a radio receiver connected to electric antennas (Figure 2).

Figure 2. . Impulsions de champ électrique mesurées par une antenne électrique de l’instrument RPWS sur la sonde Cassini à environ 1.5 Unités Astronomiques du soleil (crédit : Schippers et al 2015, ApJ sous presse)
Figure 2. . Impulsions de champ électrique mesurées par une antenne électrique de l’instrument RPWS sur la sonde Cassini à environ 1.5 Unités Astronomiques du soleil (crédit : Schippers et al 2015, ApJ sous presse)

Figure 2. Electric pulses measured by an electric antenna of the RPWS instrument on the Cassini spacecraft at about 1.5 AU from the Sun (credit : Schippers et al 2015, ApJ in press).

The analysis of the spectral power density of the voltage measured at several distances from the Sun has revealed how the nanodust flux decreases with heliocentric distance (Figure 3).

Figure 3. Flux de nano poussières mesuré par l’instrument RPWS/HFR à bord de la sonde Cassini entre les orbites de la Terre et de Jupiter. Pour comparaison, les symboles en couleur vers 5 UA représentent des mesures faites précédemment près de Jupiter (crédit : Schippers et al 2015, ApJ sous presse)
Figure 3. Flux de nano poussières mesuré par l’instrument RPWS/HFR à bord de la sonde Cassini entre les orbites de la Terre et de Jupiter. Pour comparaison, les symboles en couleur vers 5 UA représentent des mesures faites précédemment près de Jupiter (crédit : Schippers et al 2015, ApJ sous presse)

Figure 3. Nanodust flux measured by the RPWS/HFR radio receiver onboard Cassini between the orbits of Earth and Jupiter. For comparison purposes, the color symbols near 5 AU show previous measurements near Jupiter (credit : Schippers et al 2015, ApJ in press).

This measured flux is in agreement with the release of nanodust particles near the Sun and their acceleration to speeds as high as 400 km/s by the electric field of the magnetized solar wind, as predicted by models of nanodust dynamics. Far enough from Jupiter, these nanoparticles originating in the inner heliosphere outnumber largely those ejected by this planet.

Credits : The Principal Investigator of the RPWS instrument is W. S. Kurth, who replaced D. Gurnett in 2014 (Université d’Iowa, USA). The receiver RPWS/HFR was designed and built in LESIA (Observatoire de Paris, INSU-CNRS, Universities Paris 6 and Paris 7) with the support of CNES and CNRS.

References

  • Schippers P., Meyer-Vernet N., Lecacheux, A., Belheouane, S., Moncuquet, M., Mann, I., Kurth W. S., Mitchell D. G., André, N. : 2015, Nanodust detection between 1 and 5 AU by using Cassini wave measurements, Astrophys. J., in press,
  • Schippers P., Meyer-Vernet N., Lecacheux, A., Kurth W. S., Mitchell D. G., André, N. : 2014) Nanodust detection near 1 AU from spectral analysis of Cassini/Radio and Plasma Wave Science data, Geophysical Research Letters, 41, 5382-5388, DOI : 10.1002/2014GL060566

Scientific contact

Actualités